Found this little story of a programming gotcha at Peter Golde's Weblog via Cyrus's Weblog (he's a VS.NET Core guy at Microsoft). It talks about the dangers of having immutable objects within a struct (which is mutable)... Here is the code snippet that caused him issues:

struct FooPart
{
   // other data (which was immutable)
   Pen pen;

   public void ReleasePens()
   {
      if ( pen != null )
      {
         pen.Dispose();
         pen = null;
      }
   }
}

class Foo
{
   FooPart[] parts;

   public void ReleasePens()
   {
      foreach ( FooPart part in parts )
      {
         part.ReleasePens();
      }
   }
}

So what's wrong with this code? I actually did figure this out as soon as I saw the code... but it's a good reminder anyway. Go to Peter Golde's Weblog for the answer and explanation.